Effect of oxalate and pH on photodegradation of pentachlorophenol in heterogeneous irradiated maghemite System

Article abstract of DOI:10.1016/j.jphotochem.2016.06.001

Photochemical degradation in the system of iron oxides and oxalic acid (OX) is the important reaction for detoxification of organic pollutants in natural environments, including surface soils, surface water, and even aerosols, and it was more effective at low pH according to previous studies. However, in this study, the photodegradation of pentachlorophenol (PCP) proceeded rapidly at different pH conditions in the system with maghemite and OX under UV light illumination. It was observed that the removal of PCP was 77.7% ± 0.90%, 79.9% ± 0.80% and 74.3% ± 1.50% at initial pH of 3.5, 5.0 and 7.0, respectively. To explore the degradation mechanism, the interaction of OX and maghemite were systematically studied as a function of pH. The presence of OX of 1.2 mM effectively decreased the iso-electric point (iep) of the maghemite from 5.6 to 1.8. The maximum adsorption amount of maghemite adsorbing OX increased with increasing pH value from 208 mmol kg^-1 at pH = 3.5 to 293 mmol kg^-1 at pH = 9.0. However, PCP (0.0375 mM) inhibited the adsorption of oxalic acid at pH = 3.5 and pH = 5.0 but promoted it at pH = 7.0 and pH = 9.0. When the initial content of OX was 1.2 mM, the highly active compounds of Fe(C₂O₄)₃^3- as Fe(III) and Fe(C₂O₄)₂^2- as Fe(II) were the dominant species at different pH. The formation of H₂O₂ also relied on the value of pH and the concentration range of H₂O₂ during PCP degradation was 0-1.67 mg L^-1, 0-1.16 mg L^-1 and 0-0.16 mg L^-1at initial pH of 3.5, 5.0 and 7.0, respectively. The low pH conditions favored the iron cycling, the H₂O₂ generation and the broken of aromatic ring of PCP, so as to enhance the degradation rates of PCP. At the high pH conditions, due to the slowdown of the iron cycling and the decreased amount of H₂O₂ formation, the direct photolysis was responsible for the enhanced degradation of PCP. The foundation of high photochemical efficiencies of OX and maghemite for PCP degradation at large-scale pH conditions improves the photochemical mechanisms of OX-iron oxide system and is of important for understanding the transformation of organic pollutants in light environments.

Full text of DOI:10.1016/j.jphotochem.2016.06.001

Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effect of oxalate and pH on photodegradation of pentachlorophenol in
heterogeneous irradiated maghemite System
Qing Lana,b, Meiyuan Caoa, Zhijun Yea, Jishu Zhub, Manjia Chenb, Xuequan Chena,
Chengshuai Liub,
*
a
Environment Monitoring Department, Guangdong Polytechnic of Environmental Protection Engineering, Foshan, Guangdong 528216, China
Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong Institute of Eco-Environmental and Soil Sciences,
b
Guangzhou 510650, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Photochemical degradation in the system of iron oxides and oxalic acid (OX) is the important reaction for
detoxification of organic pollutants in natural environments, including surface soils, surface water, and
even aerosols, and it was more effective at low pH according to previous studies. However, in this study,
the photodegradation of pentachlorophenol (PCP) proceeded rapidly at different pH conditions in the
system with maghemite and OX under UV light illumination. It was observed that the removal of PCP was
77.7% ꢀ 0.90%, 79.9% ꢀ 0.80% and 74.3% ꢀ 1.50% at initial pH of 3.5, 5.0 and 7.0, respectively. To explore the
degradation mechanism, the interaction of OX and maghemite were systematically studied as a function
of pH. The presence of OX of 1.2 mM effectively decreased the iso-electric point (iep) of the maghemite
from 5.6 to 1.8. The maximum adsorption amount of maghemite adsorbing OX increased with increasing
pH value from 208 mmol kgꢁ1 at pH = 3.5 to 293 mmol kgꢁ1 at pH = 9.0. However, PCP (0.0375 mM)
inhibited the adsorption of oxalic acid at pH = 3.5 and pH = 5.0 but promoted it at pH = 7.0 and pH = 9.0.
When the initial content of OX was 1.2 mM, the highly active compounds of Fe(C2O4)33ꢁ as Fe(III) and Fe
(C2O4)22ꢁ as Fe(II) were the dominant species at different pH. The formation of H2O2 also relied on the
Received 7 February 2016
Received in revised form 25 May 2016
Accepted 3 June 2016
Available online 7 June 2016
Keywords:
Iron oxides
PCP
Oxalic acid
Degradation mechanism
pH condition
value of pH and the concentration range of H2O2 during PCP degradation was 0–1.67 mg Lꢁ1
,
0–1.16 mg Lꢁ1 and 0–0.16 mg Lꢁ1at initial pH of 3.5, 5.0 and 7.0, respectively. The low pH conditions
favored the iron cycling, the H2O2 generation and the broken of aromatic ring of PCP, so as to enhance the
degradation rates of PCP. At the high pH conditions, due to the slowdown of the iron cycling and the
decreased amount of H2O2 formation, the direct photolysis was responsible for the enhanced degradation
of PCP. The foundation of high photochemical efficiencies of OX and maghemite for PCP degradation at
large-scale pH conditions improves the photochemical mechanisms of OX-iron oxide system and is of
important for understanding the transformation of organic pollutants in light environments.
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
In this system, many refractory organics can be degraded
efficiently by the attack of ꢂOH [2–6]. According to previous
Iron oxide (IO) and oxalic acid are ubiquitous in the environ-
ment. Iron is the fourth most abundant element of the earth’s crust.
Oxalic acid, mainly secreted by plant roots or formed by
incomplete combustion of hydrocarbons, occurs widely in atmo-
sphere, water and soil [1–3]. So, sunlight, iron oxide and oxalic acid
can establish a heterogeneous photo-Fenton-like system in nature
[2–6].
studies, the main degradation mechanisms include several key
processes. Firstly, by the adsorption of oxalic acid on the iron oxide,
the Fe-oxalate complexes are formed [5–7]. Secondly, light
illumination greatly enhance the reductive dissolution of
Fe(III)-oxalate complexes with the formation of oxalate radical
(C2O4)ꢂꢁ and Fe(II) [2,3,5–7]. Thirdly, in the presence of O2 and
ꢂꢁ
light, C2O4 induce the generation of a series of active radicals
including CO2ꢂꢁ, O2ꢂꢁ and ꢂOOH which cause the formation of H2O2
in situ. The H2O2 react with ꢂFe(II) by Fenton reaction to generate
the most reactive radical of OH. [2–6].
* Corresponding author at: Guangdong Institute of Eco-Environmental and Soil
Sciences, No. 808, Tianyuan Road, Guangzhou 510650, China.
E-mail address: csliu@soil.gd.cn (C. Liu).
In this heterogeneous system, oxalic acid is one of important
reagent and is also degraded effectively by the attack of ꢂOH.
http://dx.doi.org/10.1016/j.jphotochem.2016.06.001
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
199
Furthermore, oxalic acid, as a short chain carboxylic acid, is easily
biodegradable [8]. In practical treatment, the heterogeneous
photo-Fenton-like system does not easily bring about the
secondary contamination of the abundant dissolved Fe. Generally,
good degradation efficiency occurs at low pH in this system [2–6].
The pH is an important factor for the activity of this system
because it affects not only the oxalic acid adsorption process, but
also the rate of iron cycling and the proportion of Fe speciation
during photochemical processes [2,3,5–7]. However, the system-
atic influence of pH on the above aspects is not very clear especially
for the iron oxide of maghemite (g-Fe2O3). The g-Fe2O3 has
magnetic properties which is favorable to recycling if the system is
applied to the practical waste treatment.
Pentachlorophenol (PCP) is the most toxic chlorophenol and
was commonly used as the wood preservative, herbicide and
pesticide in the past [8,9]. Its residues can be widely found in the
river water, soil, sediments and groundwater [8–13]. Due to its high
toxicity and environmental stability, much attention has been
focused on the removal of its residues from the environment
[10–13]. However, the direct photolysis of PCP might form more
toxic products including polychlorinated dibenzo-p-dioxins and
polychlorinated dibenzofurans [14–16]. The biodegradation of PCP
and the reduction of PCP by Fe(II)-based or zero-valence iron
technology occurs quite slowly [13,17,18].
Fig. 1. The XRD patterns and d-spacing values of g-Fe2O3.
the
g
-Fe2O3 was analyzed to be 14.36 m2 gꢁ1 by the Brunauer–
Emmett–Teller (BET) method using the Micromeritics (ASAP-
2010M, USA).
In this study, PCP as a target was chosen to systematically study
the pH effect on the UV/
process and photochemical process. For the adsorption process,
the zeta-potential of -Fe2O3, the adsorption of oxalic acid and PCP
on -Fe2O3 at different pH were investigated. For the photochemi-
cal process, during the degradation of PCP at different initial pH,
the reaction kinetics of pH, oxalic acid, PCP, the dissolution of
g
-Fe2O3/OX system including adsorption
2.2. Photochemical and adsorption experiments
g
The stock solution of aqueous PCP (0.075 mM) was prepared as
previously reported [2]. In photochemical experiments, the initial
PCP concentration was 0.0375 mM with the dosage of 0.4 g Lꢁ1 of
g
g
-Fe2O3 and the reaction temperature was performed at 30 ꢃC in all
g
-Fe2O3, the Fe speciation distribution, the in situ generation of
photodegradation experiments. The NaOH or HClO4 was used to
adjust the initial pH of suspensions. The photochemical reactor
system and the procedure of photochemical experiments were
described in detail previously [3]. An 8 W LZC-UV lamp (Luzchem
Research, Inc.) with the main emission at 350 nm and a light
intensity of 1.2 mW cmꢁ2 was used as a UV light source. Samples
were withdrawn at specific time intervals and analyzed immedi-
ately after centrifugation at 4500 rpm for 25 min.
H2O2 and the intermediate products of PCP were researched in
detail. The results of this research are helpful to further understand
the degradation mechanisms of this system and may shed light on
the practical application of this system on the degradation of
photosensitive pollutants such as PCP. In addition, the application
of the calculation method of Fe speciation at different pH in this
paper may provide a useful reference for other researches.
The adsorption experiments were carried out in Erlenmeyer
flasks containing various concentrations of oxalic acid or PCP and
2. Experiments and methods
the same dosage of g
-Fe2O3 of 0.4 g Lꢁ1. In a thermostatic shaker,
2.1. Chemicals
the suspensions were stirred at 180 rpm for 24 h with a constant
temperature (30 ꢀ1 ꢃC). The suspensions were filtered through
PCP (98%) manufactured by Aldrich, USA, was used. The organic
solvents of methanol (HPLC grade) and hexane (HPLC grade), and
the standard substance of tetrachloro-o-benzoquinone (97%) and
tetrachloro-p-benzoquinone (99%) were purchased from Acros,
Belgium. Other chemicals with an analytical grade were obtained
from Guangzhou Chemical Reagent Factory, China. All the
chemicals were used without further purification except acetic
anhydride, which was redistilled to derive PCP for intermediates
analysis. Deionized water was used in all experiments.
0.45 mm filters (MEMBRANA, Germany) and the filtrates were
analyzed for the concentrations of oxalic acid or PCP. The effect of
filters on the concentration test of PCP or oxalic acid was less than
4% that is acceptable in analysis.
2.3. Analysis
PCP concentrations were determined by HPLC (Waters
1525/2487) with an Xterra C18 reverse-phase column manufac-
tured by Waters, USA. The mobile phase containing 1% acetic acid
in water/methanol (20:80 v/v) mixed solvent flowed at a rate of
1.0 mL minꢁ1. The UV detection was set at 295 nm and the column
temperature was set at 35 ꢃC.
The intermediates of PCP were identified by HPLC, GC–MS and
IC. By the same HPLC machine as described above, the two
intermediates of tetrachloro-o-benzoquinone (o-chloranil) and
tetrachloro-p-benzoquinone (p-chloranil) were confirmed by
comparing their retention times with internal standards. The
mobile phase containing 1% acetic acid in water/methanol
(25:75 v/v) mixed solvent flowed at a rate of 1.0 mL minꢁ1. The
UV detection was set at 280 nm and the column temperature was
set at 30 ꢃC.
The preparation of
g-Fe2O3 followed the method previously
reported [19]. Firstly, 100 mL of 2 M (CH2)6N4 solution, 500 mL of
0.2 M FeCl2 solution, and 100 mL of 1.25 M NaNO3 solution were
mixed to obtain a black green precipitate. Secondly, the precipitate
was aged in the mixture at 80 ꢃC for 6 h before it was filtered by the
0.45 mm glass fiber paper. And the precipitate remained on the
filter paper was washed three times with alcohol and distilled
water to remove anions and organic impurities and then dried at
60 ꢃC for 60 h. Finally, the dried gel was ground and
g-Fe2O3
powders were obtained. The X-ray powder diffraction (XRD)
(RigakuD/Max-III, Japan) was used to confirm the crystal structure
as shown in Fig. 1. By comparing with standard XRD pattern, the
pure phase of g-Fe2O3 was confirmed. The specific surface areas of

200
Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
By GC–MS (Thermo Trace-DSQ-2000) system with an Agilent
UV / IO / pH 3.5
(A)
silicon capillary column (0.25 mm ꢄ 30 m) and electron ionization,
the two intermediates of 2,3,5,6-tetrachloro-1,4-hydroquinone
(TeCHQ) and tetrachlorocatechol (TeCC) were identified. Before
GC/MS analysis, the samples were derived by acetic anhydride as
references [13].
The other two intermediates of HCOOH and CH3COOH were
identified by LC-10A system (Shimadzu) with IC-A3 column. The
column temperature was set at 40 ꢃC and the flow rate was set at
1.2 mL minꢁ1. The mobile phase was the mix solution of 2.4 mM
Tris and 2.5 mM phthalic acid.
1.0
0.8
0.6
0.4
0.2
UV / OX / pH 3.5
UV / OX / IO / pH 3.5
UV / OX / IO / pH 5.0
UV / OX / IO / pH 7.0
0
20
40
40
40
60
The concentrations of oxalic acid were determined by ion
chromatography (IC Dionex ICS-90) with a Dionex IonPac AS14A
column (250 ꢄ 4.6 mm, Dionex, USA). The flow rate was 1.0 mL
minꢁ1 and the mobile phase was the mix solution of 8.0 mM
Na2CO3 and 1.0 mM NaHCO3.
1.0
0.8
0.6
0.4
0.2
7.5
The zeta-potentials of g-Fe2O3 at different pH were measured
using a zeta potential analyzer (ZetaPlus2002) manufactured by
Brookhaven Inc., USA. The analysis procedures were the same as
the previous study [6].
Dissolved Fe(II) was measured by the ferrozine method
described in detail in our previous publications [17,19]. After
adding 10% OHNH3Cl to reduce all Fe(III) to Fe(II), the total
dissolved iron was determined. By subtracting the amount of
dissolved Fe(II) from the total amount of dissolved iron, the
dissolved Fe(III) content can be got.
(B)
0
(C)
20
60
The content of H2O2 generated in situ in aqueous solution was
measured using an H2O2 analyzer (Lovibond-ET8600, Germany) as
described in our previous publications [2,6].
6.0
4.5
3.0
3. Results and discussion
3.1. Photodegradation of PCP
0
20
reaction time (min)
60
As showed in Fig. 2, under UV irradiation only, PCP was
photolyzed by 54.6 ꢀ1.70%, 65.5% ꢀ 1.14% and 70.2% ꢀ 2.20% at pH
of 3.5, 5.0 and 7.0, respectively. It was found that the removal of PCP
increased with the increase of pH. This finding was in agreement
with the result of Benitez et al. [20]. The reason is that the
absorbance of PCP increases when the pH is increased [20]. Under
this condition, the degradation of PCP was due to the direct
photolysis as the following equation:
Fig. 3. The variations of PCP (A) and oxalic acid (OX) (B), as well as pH value (C) over
the photodegradation of PCP in -Fe2O3 system at different initial pH.
g
3.5, under UV/g-Fe2O3 and UV/OX, the removal of PCP was close by
57.6% ꢀ 2.00% and 59.6% ꢀ 3.60%, respectively, which was similar
with that under UV only. Under these conditions, the degradation
of PCP was mainly resulted from the direct photolysis as Eq. (1).
PCP + hv ! PCPoxid
(1)
Fig. 3 shows the variation of PCP and oxalic acid, as well as the
pH change during the photodegradation of PCP with the same
initial oxalic acid concentration (C0ox) of 1.2 mM. At the initial pH of
Under heterogeneous UV/g-Fe2O3/OX system, at initial pH of
3.5, the degradation of PCP increased significantly to 77.7% ꢀ 0.90%.
It should be attributed to the attack of ꢂOH which was mainly
formed by the following equations initiated by the photoreduction
of Fe(III)-oxalate complexes. The Fe(III)-oxalate complexes were
1.0
UV only / pH 3.5
formed by the adsorption of oxalic acid on the surface of
g
-Fe2O3.
UV only / pH 5.0
UV only / pH 7.0
0.8
hv
3ꢁ2n
2ꢁ2n
ꢂꢁ
FeIIIðC2O4Þn ! FeIIðC2O4Þn
þ C2O4
ð2Þ
k = 0.04 sꢁ1 [21]
0.6
0.4
0.2
0.0
hv
2þ
FeIIIðOHÞ ! FeðIIÞ þ ꢂOH
ð3Þ
ð4Þ
ð5Þ
ð6Þ
k = 0.0012 sꢁ1 [21]
ꢂꢁ
C2O4 ! CO2 þ COꢂ2ꢁ
k = 2 ꢄ106 sꢁ1 [22]
CO2ꢂꢁ þ O2 ! CO2 þ Oꢂ2ꢁ
k = 2.4 ꢄ109 Mꢁ1 sꢁ1[23]
ꢂOOH$Oꢂ2ꢁ þ Hþ
0
20
Reaction Time (min)
Fig. 2. The variation of PCP under UV irradiation only at different pH.
40
60

Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
201
k = 1.58 ꢄ 10ꢁ5 M [24]
simultaneously happened in solution and on the surface of iron
oxide. Because the surface of stirrer is little, not all -Fe2O3 were
sucked on the stirrer and there were chances to touch and degrade
PCP among -Fe2O3, oxalic acid and PCP in solution and on the
surface of -Fe2O3.
For further understanding the reaction mechanisms of PCP in
the heterogeneous system at different pH, the adsorption of oxalic
acid and PCP, the Fe species, the formation of H2O2 and the
intermediates of PCP were researched as following sections.
g
ꢂOOH þ Oꢂ2ꢁ þ Hþ ! H2O2 þ O2
k = 9.7 ꢄ107 Mꢁ1 sꢁ1 [24]
ꢂOOH þ ꢂOOH ! H2O2 þ O2
ð7Þ
ð8Þ
g
g
k = 8.3 ꢄ105 Mꢁ1 sꢁ1 [24]
3ꢁ2n
3ꢁ2n
FeIIðC2O4Þn
þ H2O2 ! FeIIIðC2O4Þn
þ ꢂOH þ OHꢁ
ð9Þ
k = 3.1 ꢄ104 Mꢁ1 sꢁ1 [25]
3.2. Adsorption of oxalic acid and PCP on the
g-Fe2O3
2ꢁ
ꢂOH þ ðC2O4Þ ! CO2 þ COꢂ2ꢁ þ OHꢁ
ð10Þ
ð11Þ
The zeta-potentials of
oxalic acid were measured as shown in Fig. 4. In the absence of
oxalic acid, the isoelectric point (iep) of -Fe2O3 was at pH 5.6. This
iep value is in generally accorded with the previously reported
value of 6.6 [27] and lower than that of -Fe2O3 or -FeOOH [6].
However, in the presence of oxalic acid, the iep of -Fe2O3
decreased sharply at a much lower pH value of 1.8, which is
consistent with the behavior of -Fe2O3 and -FeOOH [6,7]. The
g-Fe2O3 in the absence and presence of
k = 7.7 ꢄ106 Mꢁ1 sꢁ1 [25,26]
ꢁ
ꢂOH þ HC2O4 ! CO2 þ COꢂ2ꢁ þ H2O
g
k = 4.7 ꢄ107 Mꢁ1 sꢁ1 [25,26]
a
a
ꢂꢁ
According to above reactions, C2O4 can be formed by the
photoreduction of Fe(III)-oxalate complexes as Eq. (2) and induce
g
the generation of a series of active radicals including CO2ꢂꢁ, O2
ꢂꢁ
a
a
and ꢂOOH as Eq. (4)–(6) which cause the formation of H2O2 in situ
shift was resulted from the formation of Fe-oxalate complexes and
the release of OHꢁ during the adsorption process of oxalic acid on
the iron oxides as follows [28]:
by the reaction of ꢂOOH and O2ꢂꢁ as Eqs. (7) and (8). The H2O2 react
with Fe(II) by Fenton reaction to generate the most reactive radical
of ꢂꢂꢁOH as Eq. (9). Simultaneously, Fe(III) was reduced by CO2
,
ꢂꢁ
ꢅ Fe ꢁ OH þ H2C2O4$ ꢅ Fe ꢁ H2C2O4 þ OHꢁ
(The main reaction occur in pH < 1.25)
ð12Þ
O2 and ꢂOOH to generate Fe(II), Fe(II) was oxidized by O2ꢂꢁ and
ꢂOOH to generate Fe(III). So, the iron cycling was set up. The
relevant mechanisms reactions are dicussed in Section 3.3.
ꢅ Fe ꢁ OH þ HC2O4ꢁ$ ꢅ Fe ꢁ HC2O4 þ OHꢁ
(The main reaction occur in 1.25 < pH < 4.21)
ð13Þ
Furthermore, in UV/g-Fe2O3/OX system at initial pH 3.5, the
changing inflection point of PCP concentration, oxalic acid
concentration, and the pH value all occurred at 40 min as shown
in Fig. 3. After 40 min, the pH increased above 6 and remained
stable, the degradation rate of PCP and oxalic acid also slowed
down obviously. It indicates that the same reaction mechanism
dominate the degradation of PCP and oxalic acid at initial pH 3.5.
In fact, oxalic acid can be effectively degraded by ꢂOH at a high
rate of 106–107 Mꢁ1 Sꢁ1 as Eqs. (10) and (11) [25,26]. A higher
ꢅ Fe ꢁ OH þ C2O42ꢁ$ ꢅ Fe ꢁ C2O4 þ OHꢁ
ð14Þ
(The main reaction occur in pH > 4.21)
So, the shift in iep clearly indicated the adsorption of oxalic acid
-Fe2O3 was a chemical adsorption and the surface of -Fe2O3
on
g
g
were negative charge over a wide pH range of 2–9 in the presence
of oxalic acid as shown in Fig. 4, which greatly influenced the
ꢂ
amount of OH indicates a more rapid degradation of oxalic acid.
surface properties of the
To further explore the adsorption of oxalic acid on the surface of
-Fe2O3, the adsorption isotherms of oxalic acid on -Fe2O3 were
g-Fe2O3.
The degradation of oxalic acid was rapid at initial pH 3.5 but slowed
down obviously at pH 5.0 and was little at pH 7.0. It implied that the
attack of ꢂOH is mainly responsible for both the degradation of PCP
and oxalic acid at initial pH 3.5.
g
g
studied as presented in Fig. 5. It was found that the Langmuir
model described the isotherms well as marked by the solid lines.
At higher initial pH of 5.0 and 7.0 in UV/g-Fe2O3/OX system, the
removal of PCP was 79.9% ꢀ 0.80%, and 74.3% ꢀ 1.50%, respectively,
which was close to that at initial pH 3.5 and did not decreased with
80
the increase of pH that happened in UV/a-Fe2O3/OX and UV/
γ-Fe O
2 3
a
-FeOOH/OX system [6]. Furthermore, this behavior reappeared
under C0ox = 0.8 mM (data not presented) in UV/
system.
g-Fe2O3/OX
Iron oxide alone
Iron oxide with oxalic acid
In the heterogeneous UV/OX/IO system, PCP can be degraded
simultaneously by the attack of ꢂOH and the direct photolysis. The
formation of ꢂOH decrease when the pH is increased indicated by
the degradation rate of oxalic acid and other studies [6,17,19–21].
However, the direct photolysis of PCP is enhanced with the
40
0
increase of pH as shown in Fig. 2. In UV/
UV/ -FeOOH/OX system, the turbidity of iron oxides inhibited
the UV penetration [6]. In UV/ -Fe2O3/OX system, however, the
reaction solution was clearer compared with that in -Fe2O3 or
-FeOOH system because many -Fe2O3 have been sucked on the
magnetic stirrer which is used to stir suspension continuously.
Therefore, in the -Fe2O3 system, the powder of -Fe2O3 had little
a-Fe2O3/OX and
a
g
a
a
g
-40
g
g
effect on the penetration of light, which was confirmed by the data
that the removal rates of PCP were similar in UV irradiation only,
2
4
6
8
UV/
g
-Fe2O3 and UV/OX as described above. Thus, the removal rates
-Fe2O3/OX
pH
of PCP did not decrease with the increase of pH in UV/
g
system because increased pH favored the direct photolysis of PCP.
Furthermore, in such a heterogeneous system, the reactions
Fig. 4. The zeta-potentials of
g-Fe2O3 in the absence and presence of oxalic acid
(1.2 mM) at different pH.

202
Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
0.0375 mM, the adsorbed PCP could be desorbed by centrifugation.
400
300
200
100
0
As the samples were analyzed after centrifugation during the
photochemical experiments, the removal of PCP should be
attributed to the photoreaction rather than the adsorption.
By comparison, at different pH in the absence and presence of
PCP, the maximum adsorption amount of oxalic acid on iron oxide
all follows the sequence:
in Fig. 7. The synthesized method of
described in our previous study [17]. At initial pH 3.5 in UV/IO/OX,
the removal of PCP was 83.3%, 77.7% and 68.0% for -Fe2O3,
-Fe2O3, and -FeOOH, respectively, which also followed the
a
-Fe2O3 >
g-Fe2O3 >
a
-FeOOH as shown
a
-Fe2O3 and
a-FeOOH were
pH = 3.5
pH = 5
pH = 7
pH = 9
a
g
a
sequence of their ability of adsorbing oxalic acid as described
above. Since the photoactive Fe(III)-oxalate complexes are formed
by the oxalic acid adsorption process and directly contribute the
generation of active species (H2O2 and ꢂOH), the consistent
sequence confirmed the contribution of adsorption process for the
activity of IO in this system to a certain extent.
(A) OX only
0.8
(B) OX with 0.0375 mM PCP
1.6 0.0 0.4 0.8 1.2 1.6
0.0
0.4
1.2
Equilibrium concentration of oxalic acid (mM)
Fig. 5. The adsorption isotherms of oxalic acid on g-Fe2O3 at different pH.
3.3. Formation of Fe species during PCP photodegradation at different
initial pH
Also, pH significantly influenced the adsorption of oxalic acid on
-Fe2O3. The adsorption amount of oxalic acid increased with
increasing pH and there were close value in pH 7.0 and pH 9.0,
g
To further explore the pH effect on the photochemical process,
the proportion of various Fe species during the PCP photo-
degradation at different initial pH were calculated by the method
previously reported [2,29] and the results were shown in Fig. 8, and
the amounts of dissolved Fe were measured as shown in Fig. 9.
Fig. 8 shows the fraction of different Fe(III)/Fe(II) species during
PCP degradation. The Fe(III) species include Fe3+, FeC2O4+, Fe
(C2O4)2ꢁ Fe(C2O4)33ꢁ and FeHC2O42+. The Fe(II) species include Fe2
which is in agreement with the behavior of a-Fe2O3 and a-FeOOH
[6]. In this system, the three species of H2C2O4, HC2O4ꢁ, C2O42ꢁ are
the speciation of oxalic acid in solution and the four Fe(ꢁIII)-oxalate
2+
+
complexes, namely FeHC2O4
, FeC2O4 , Fe(C2O4)2 and Fe
(C2O4)33ꢁ, can be formed accordingly in the presence of Fe(III).
The stability of Fe(III)-oxalate complexes increase with the
2ꢁ
2+
increasing coordination number of C2O4
as follows: Fe
because their
+
(C2O4)33ꢁ >> Fe(C2O4)2ꢁ >> FeC2O4 ꢆ FeHC2O4
+, Fe(C2O4)2 and Fe(C2O4)34ꢁ. It should be noted that these Fe
2ꢁ
dissociation constants are 3 ꢄ10ꢁ21, 6.31 ꢄ10ꢁ17, 3.98 ꢄ 10ꢁ10
,
species exist simultaneously both in solution and on the surface of
and 2.95 ꢄ10ꢁ10, respectively [29]. As the fraction of C2O42ꢁ in the
g
-Fe2O3. The lower pH favored the dissolution of iron oxide and
more Fe(III) species can be released to solution from the surface of
-Fe2O3 at pH 3.5. Fig. 9 shows the amount of the dissolved
solution just increases with increasing pH, it indicated that at
2ꢁ
higher pH, there are more C2O4 to form more stable Fe(III)-
g
oxalate complexes that are difficult to desorb. When pH > 6.0,
Fe(III)/Fe(II) species including different Fe species.
2ꢁ
C2O4
becomes the sole species and the amounts of Fe(III)-
-Fe2O3 were close at pH 5.0 and pH 9.0.
In the UV/IO/OX system, the Fe speciation and the iron cycling is
very important to the photochemistry activity of system [30]. The
Fe(III)-oxalate complexes with photochemistry activity are formed
by adsorption as Eqs. (12)–(14) and are reduced to form Fe(II)
speciation in illumination as Eq. (2) and Eqs. (15)–(17), in which Fe
(II) reacts with O2ꢂꢁ and ꢂOOH to form H2O2 and Fe(II) itself is re-
oxidized to Fe(III) in the presence of O2 as Eqs. (20)–(22)
[21,25,28,31–35]. So the iron redox cycling involving a series of
oxalate formed on
g
Moreover, as shown in Fig. 5, PCP (0.0375 mM) inhibited the
adsorption of oxalic acid at low pH but promoted it at high pH,
which is similar to that of a-Fe2O3 [6]. And adsorption experiments
of PCP in the presence of 1.2 mM oxalic acid at different pH values
were also conducted as shown in Fig. 6. Overall, the adsorption of
PCP can be described by the Freundlich model well, implying that
ꢂ
the adsorption mechanisms of PCP on
g-Fe2O3 should owe to
active radicals and the formation of H2O2 and OH can be set up.
electrostatic adsorption rather than chemical adsorption. More-
over, with the increase in pH, the adsorption of PCP increased. In
the experimental concentration range of PCP from 0.00375 to
However, the activity of different Fe speciation is varied. Fe
(C2O4)33ꢁ and Fe(C2O4)2ꢁ can be more efficiently photolyzed than
other Fe(III) species as Eq. (2–3) and Eq. (17) [21,31–33]. Fe
60
50
40
30
20
pH = 3.5
10
0
pH = 5.0
pH = 7.0
0
5
10
15
20
25
Equilibrium concentration of PCP (μΜ
)
Fig. 6. The adsorption isotherms of PCP on
g
-Fe2O3 at different pH.
Fig. 7. The maximum adsorption amount of oxalic acid on different iron oxides.

Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
203
Fig. 8. The Fe speciation during PCP degradation with C0ox = 1.2 mM at different initial pH. (A) Fe(III); (B) Fe(II).
(C2O4)22ꢁ as Fe(II) has a much faster rate than Fe2+ in reacting with
H2O2 to form ꢂOH as Eq. (9) and Eq. (22) [25,35]. The involved
reactions are listed as following:
0.020
0.016
0.012
0.008
0.004
0.000
0.020
0.016
0.012
0.008
0.004
0.000
(A)
(B)
hv
3ꢁ2n
2ꢁ2n
ꢂꢁ
FeIIIðC2O4Þn ! FeIIðC2O4Þn
þ C2O4
ð2Þ
pH 3.5
pH 5.0
pH 7.0
k = 0.04 sꢁ1 [21]
hv
FeIIIðOHÞ2þ! FeðIIÞ þ ꢂOH
ð3Þ
ð9Þ
k = 0.0012 sꢁ1 [21]
3ꢁ2n
3ꢁ2n
FeIIðC2O4Þn
þ H2O2 ! FeIIIðC2O4Þn
þ ꢂOH þ OHꢁ
k = 3.1 ꢄ104 Mꢁ1 sꢁ1 [25]
3ꢁ2n
2ꢁ2n
FeIIIðC2O4Þn
þ O2ꢂꢁ ! FeIIðC2O4Þn
þ O2
ð15Þ
ð16Þ
ð17Þ
k = 1 ꢄ105 Mꢁ1 sꢁ1 [25]
3ꢁ2n
FeIIIðC2O4Þn
k = 1.2 ꢄ104 Mꢁ1 sꢁ1 [25]
2ꢁ2n
þ ꢂOOH ! FeIIðC2O4Þn
þ O2 þ Hþ
þ CO2
0
20
40
60
0
20
40
60
Reaction time (min)
3ꢁ2n
FeIIIðC2O4Þn
2ꢁ2n
þ CO2ꢂꢁ ! FeIIðC2O4Þn
Fig. 9. The variation of dissolved Fe(III) (A) and Fe(II) (B) during the PCP
photodegradation at different initial pH.

204
Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
k ꢆ 8 ꢄ 109 Mꢁ1 sꢁ1 [33]
Fe3þ þ O2ꢂꢁ ! Fe2þ þ O2
reductive dissolution of Fe(III)-oxalate complexes as illustrated
in B column of Fig. 9. It is obvious that lower pH benefited the
dissolution of -Fe2O3, which is similar to that of -Fe2O3 and
-FeOOH [6]. But in -Fe2O3 system, the amounts of dissolved Fe
were in the range of 0.001–0.018 mM for Fe(III) and
0.001–0.011 mM for Fe(II), respectively, which were much less
ð18Þ
ð19Þ
ð20Þ
g
a
k = 1.5 ꢄ108 Mꢁ1 sꢁ1 [21]
Fe3þ þ ꢂOOH ! Fe2þ þ O2 þ Hþ
k = 3.3 ꢄ105 Mꢁ1 sꢁ1 [34]
FeðIIÞ þ O2ꢂꢁ þ 2Hþ ! FeðIIIÞ þ H2O2
k = 7.2 ꢄ108 Mꢁ1 sꢁ1 [33]
a
g
than those in
a-Fe2O3 and a-FeOOH system [6]. The previous
researches have reported that the dissolved Fe was not the critical
factor to determine the photocatalytic activity of iron oxide in
heterogeneous system [6,37]. So, it is not surprising that though
the dissolved Fe was little in
g-Fe2O3 system, the removal of PCP
Hþ
was higher than that in -FeOOH system [6]. So, it indicated that
a
FeðIIÞ þ ꢂOOH ! FeðIIIÞ þ H2O2
ð21Þ
ð9Þ
the adsorbed Fe(III)/Fe(II) species and/or the surface structure of
iron oxides play important roles on the removal efficiency of PCP in
the heterogeneous system. In addition, the little amount of
k = 7.2 ꢄ105 Mꢁ1 sꢁ1 [34]
3ꢁ2n
3ꢁ2n
FeIIðC2O4Þn
þ H2O2 ! FeIIIðC2O4Þn
þ ꢂOH þ OHꢁ
dissolved Fe(III)/Fe(II) in
g-Fe2O3 system was more convenient
k = 3.1 ꢄ104 Mꢁ1 sꢁ1 [25]
in wastewater treatment because it is difficult to bring about the
secondary contamination of dissolved Fe ions.
Fe2þ þ H2O2 ! Fe3þ þ ꢂOH þ OHꢁ
k = 53 Mꢁ1 sꢁ1 [35]
ð22Þ
ð23Þ
3.4. Formation of hydrogen peroxide
ꢂOH þ H2O2 ! H2O þ ꢂOOH
k = 3 ꢄ107 Mꢁ1 sꢁ1 [36]
H2O2 is a very important active species in the Fe(III)-oxalate
system. The formation of H2O2 was detected in detail as presented
It has been noted that the C0ox influenced the proportion of Fe
in Fig. 10. With UV and
60 min. In the presence of oxalic acid with UV and
g
-Fe2O3 only, no H2O2 was found during
-Fe2O3, H2O2
g
species by affecting the adsorption amount of oxalic acid on the
was detected and decrease dramatically with the increase of initial
pH values.
g
-Fe2O3 as reported in our previous study [2]. In this study at
C0ox = 1.2 mM, the maximum adsorption amount of oxalic acid can
be approached in different pH as presented in Fig. 5. Moreover, the
high activity of Fe(C2O4)33ꢁ as Fe(III) and Fe(C2O4)22ꢁ as Fe(II) were
the dominant species during the PCP photodegradation at different
initial pH as shown in Fig. 8. Nevertheless, at high pH values (>4.0),
the oxidation of Fe(II) formation becomes much faster while the
reduction of Fe(III) is hampered. Meanwhile, the iron redox cycling
originates from the photoreduction of Fe(III) [21]. It means that the
iron redox cycling would slow down and the amount of Fe(II)
would decrease at high pH. As a result, the generation of H2O2 and
ꢂOH would be inhibited because Fe(II) is indispensible to their
formation. It was confirmed by the results of H2O2 in Section 3.4.
Fig. 9 shows the variation of dissolved Fe during the PCP
photodegradation at different initial pH. In the dark, the dissolved
Fe(II) were formed by the slow reductive dissolution of Fe(III) and
the dissolved Fe(III) were generated by the desorption of surface Fe
(III)-oxalate complexes. Illumination greatly promoted the
In the studied system, the most reactive oxidant, ꢂOH, is
acquired through Fenton reaction, i.e the Fe(II) reaction with H2O2.
During the photochemical process, H2O2 is primarily formed by the
dismutation of O2ꢂꢁ/ꢂOOH and the reaction of O2ꢂꢁ/ꢂOOH with Fe
(II) as Eqs. (7) and (8) and Eqs. (20) and (21). Simultaneously, H2O2
is consumed as Eq. (9) and Eqs. (22) and (23). The content of H2O2
in the system depended on both rates of its generation and
consumption. To some extent, H2O2 concentration can be used as
an indicator to evaluate the activity of systems [2,6]. As described
in 3.3, the iron redox cycling would slow down and the generation
of H2O2 would be inhibited at high pH. Moreover, it should be
noted that by comparing the results of this paper and the previous
paper [6], the ability of iron oxides to generate H2O2 is consistent
with their ability to degrade PCP and their capacity of adsorbing
oxalic acid at pH 3.5, following the same order:
a-Fe2O3 > g-
Fe2O3 >
a-FeOOH.
1.5
1.0
0.5
0.0
3.0
pH = 3.5
pH = 5
pH = 7
pH = 3.5
pH = 5
pH = 7
2.4
1.8
1.2
0.6
0.0
0
20
40
60
0
20
40
60
Reaction time (min)
Reaction time (min)
Fig. 10. The generation of H2O2 during PCP degradation at various initial pH
Fig. 11. The accumulated concentrations of p-chloranil during PCP degradation at
conditions.
different initial pH values.

Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
205
Scheme 1. Two complementary photodegradation mechanisms of PCP in UV/g-Fe2O3/OX system at different pH.
3.5. Intermediates of PCP photodegradaion
products were generated. With the increasing of pH, the direct
photolysis gradually play a significant role to degrade PCP while
the formation of ꢂOH decreased obviously and there were no
smaller molecules products.
At initial pH 3.5, six intermediates of PCP were identified
including p-chloranil and o-chloranil by HPLC, HCOOH and
CH3COOH by IC, TeCHQ and TeCC by GC–MS. At initial pH 5 and
7, four intermediates of p-chloranil, o-chloranil, TeCHQ and TeCC
were also detected, but the ring-opening products of HCOOH and
CH3COOH were not found.The chromatograms of the detected
intermediates were similar with our previous study [6].
The ring-opening products of organic acids with small
molecules, HCOOH and CH3COOH, were the oxidation products
and their formation to some extent confirmed that the photo-
degradation of PCP was comparatꢂively thorough at pH 3.5, which
should be owing to the attack of OH.
4. Conclusions
PCP can be effectively photodegraded in UV/g-Fe2O3/OX system
at different pH. At low pH, the dominant mechanism for PCP
degradation is ꢂOH attack with the formation of higher content of
H2O2 and the ring-opening intermediates. At high pH, however, the
dominant mechanism for PCP degradation is the direct photolysis
with the formation a much lower H2O2 concentration. The capacity
of g-Fe2O3 adsorbing oxalic acid and its ability to form H2O2 were
As its relative stabilization, the intermediate of p-chloranil can
be quantified as shown in Fig. 11. At different pH, the accumulated
concentration of p-chloranil all initially increased and then
decreased, suggesting an obvious degradation process. As an
intermediate, its concentration depends on both rates of its
generation and degradation. At initial pH3.5, its accumulated
concentration was much less than that at higher initial pH and
there was an obvious increase in later reaction stage after 40 min.
This changing inflection point was consistent with that of PCP and
oxalic acid as described in section 3.1. It may be understood that at
early stage the accumulated concentration of p-chloranil at initial
pH 3.5 was less than that at high initial pH because its degradation
rate is more quicker due to a good iron cycling and the attack of
more ꢂOH. At later stage at initial pH 3.5, the increasing pH
hampered the iron cycling and the formation of ꢂOH, the
degradation rates of p-chloranil decreased and then the acumi-
nated concentration increased.
consistent with its ability of photodegrading PCP. At different
initial pH, the high activity species of Fe(III)-oxalate and
Fe(II)-oxalate were the main species at C0ox = 1.2 mM for generation
of H2O2 and ꢂOH. Moreover, the formation of dissolved Fe(III) and
Fe(II) was few in UV/
treatment effects and the low dissolved Fe species at different pH
in UV/ -Fe2O3/OX system facilitate its engineered applications to
g-Fe2O3/OX system. The highly efficient
g
treat wastewater containing photosensitive pollutants such as PCP.
Acknowledgments
The authors appreciate the financial support by the National
Natural Science Foundation of China (No. 41201503, 41301538),
Research Fund for the Doctoral Program of Higher Education (No.
20110171120025), Foundation of President of Guangdong Poly-
technic of Environmental Protection Engineering (No.
KY201301003).
Over all, as shown in Scheme 1, the PCP photodegradation was
influenced by two complementary mechanisms in UV/
g
-Fe2O3/OX
References
system. The two complementary mechanisms are heterogeneous
Fenton-like reactions and the direct photolysis of PCP. The
heterogeneous Fenton-like reactions mainly include six parts as
shown in Scheme 1. At low pH, the direct photolysis was weak
while the heterogeneous Fenton-like reactions involving iron
cycling to form more ꢂOH to attack PCP and smaller molecules
[1] Y.G. Zuo, J. Hoigne, Evidence for photochemical formation of H2O2 and
oxidation of SO2 in authentic fog water, Science 260 (1993) 71–73.
[2] Q. Lan, F.B. Li, C.S. Liu, X.Z. Li, Heterogeneous photodegradation of
pentachlorophenol with maghemite and oxalate under UVA illumination,
Environ. Sci. Technol. 42 (2008) 7918–7923.

206
Q. Lan et al. / Journal of Photochemistry and Photobiology A: Chemistry 328 (2016) 198–206
[3] F.B. Li, X.Z. Li, X.M. Li, T.X. Liu, J. Dong, Heterogeneous photodegradation of
[19] X.G. Wang, C.S. Liu, X.M. Li, F.B. Li, S.G. Zhou, Photodegradation of 2-
mercaptobenzothiazole in the (-Fe2O3/oxalate suspension under UVA light
irradiation, J. Hazard. Mater. 153 (2008) 426–433.
bisphenol A with iron oxides and oxalate in aqueous solution, J. Colloid Interf.
Sci. 311 (2007) 481–490.
[4] C.S. Liu, F.B. Li, X.M. Li, G. Zhang, Y.Q. Kuang, The effect of iron oxides and
oxalate on the photodegradation of 2-mercaptobenzothiazole, J. Mol. Catal. A
Chem. 252 (2006) 40–48.
[5] P. Mazellier, B. Sulzberger, Diuron degradation in irradiated, heterogeneous
iron/oxalate systems: the rate-determining step, Environ. Sci. Technol. 35
(2001) 3314–3320.
[6] Q. Lan, H. Liu, F.B. Li, F. Zeng, C.S. Liu, Effect of pH on pentachlorophenol
degradation in irradiated iron/oxalate systems, Chem. Eng. J. 168 (2011)
1209–1216.
[7] Y. Zhang, N. Kallay, E. Matijevic, Interactions of metal hydrous oxides with
chelating agents. VII. Hematite ꢁ oxalic and citric acid systems, Langmuir 1
(1985) 201–206.
[20] F.J. Benitez, J.L. Acero, F.J. Real, J. García, Kinetics of photodegradation and
ozonation of pentachlorophenol, Chemosphere 51 (2003) 651–662.
[21] M.E. Balmer, B. Sulzberger, Atrazine degradation in irradiated iron/Oxalate
systems: effects of pH and oxalate, Environ. Sci. Technol. 33 (1999) 2418–2424.
[22] Q.G. Mulazzani, M. D’Angelantonio, M. Venturi, M.Z. Hoffmann, M.A. Rodgers,
Interaction of formate and oxalate ions with radiation-generated radicals in
aqueous solution. Methylviologen as a mechanistic probe, J. Phys. Chem. 90
(1986) 5352–5437.
[23] K.A. Hislop, J.R. Bolton, The photochemical generation of hydroxyl radicals in
the UV-Visferrioxalate/H2O2 system, Environ. Sci. Technol. 33 (1999)
3119–3126.
[24] B.H.J. Bielski, D.E. Cabelli, R.L. Arudi, A.B. Ross, Reactivity of HO2/O2-radicals in
aqueous solution, J. PhyChem. Ref. Data 14 (1985) 1041–1100.
[25] D.L. Sedlak, J. Hoigné, The role of copper and oxalate in the redox cycling of iron
in atmospheric waters, Atmos. Environ. 27A (1993) 2173–2185.
[26] J.S. Jeong, J.Y. Yoon, pH effect on OH radical production in photo/ferrioxalate
system, Water Res. 39 (2005) 2893–2900.
[8] E. Espejo, E. Agosin, Production and degradation of oxalic acid by brown rot
Fungi, Appl. Environ. Microbiol. 57 (1991) 1980–1986.
[9] R.L. Parfitt, V.C. Farmer, J.D. Russell, Adsorption on hydrous oxides. I. Oxalate
and benzoate on goethite, J. Soil. Sci. 28 (1977) 29–39.
[10] D.F. Goerlitz, D.E. Troutman, E.M. Godsy, B.J. Franks, Migration of wood-
perserving chemicals in contaminated groundwater in a sand aquifer at
Pensacola Florida, Environ. Sci. Technol. 19 (1985) 955–961.
[11] R. Valo, V. Kitunen, M.S. Salkinoja-Salonen, S. Räisänen, Chlorinated phenols as
contaminants of soil and water in the vicinity of two Finnish sawmills,
Chemosphere 13 (1984) 835–844.
[27] L. Garcell, M.P. Morales, M. Andres-Verges, P. Tarfay, C.S. Serna, Interfacial and
rheological characteristics of maghemite aqueous solution, J. Colloid Interface
Sci. 205 (1998) 470–475.
[28] R.M. Cornell, P.W. Schindler, Infrared study of the adsorption of
hydroxycarboxylic acids on a-FeOOH and amorphous Fe(III)hydroxide, Colloid
[12] G. Engelhardt, P.R. Wallnöfer, W. Mücke, G. Renner, Transformations of
pentachlorophenol part II: transformations under environmental conditions,
Tox. Environ. Chem. 11 (1986) 233–252.
[13] J.G. Mueller, D.P. Middaugh, S.E. Lantz, P.J. Chapman, Biodegradation of
creosote and pentachlorophenol in contaminated groundwater: chemical and
biological assessment, Appl. Environ. Microbiol. 57 (1991) 1277–1285.
[14] M. Fukushima, K. Tatsumi, K. Morimoto, Influence of iron(III) and humic acid
on the photodegradation of pentachlorophenol, Environ. Toxicol. Chem. 19
(2000) 1711–1716.
Polym. Sci. 258 (1980) 1171–1175.
[29] D. Panias, M. Taxiarchou, I. Douni, I. Paspaliaris, A. Kontopoulos,
Thermodynamic analysis of the reactions of iron oxides: dissolution in oxalic
acid, Can. Metal. Quart. 35 (1996) 363–373.
[30] Q. Lan, F.B. Li, C.X. Sun, C.S. Liu, X.Z. Li, Heterogeneous photodegradation of
pentachlorophenol and iron cycling with goethite, hematite and oxalate under
UVA illumination, J. Hazard. Mater. 174 (2010) 64–70.
[31] B.C. Faust, R.G. Zepp, Photochemistry of aqueous iron(III)-polycarboxylate
complexes: roles in the chemistry of atmospheric and surface waters, Environ.
Sci. Technol. 27 (1993) 2517–2522.
[15] M. Fukushima, K. Tatsumi, Degradation pathways of pentachlorophenol by
photo-Fenton pystems in the presence of iron(III) humic acid, and hydrogen
peroxide, Environ. Sci. Technol. 35 (2001) 1771–1778.
[32] J.S. Jeong, J.Y. Yoon, pH effect on OH radical production in photo/ferrioxalate
system, Water Res. 39 (2005) 2893–2900.
[16] S. Vollmuth, A. Zajc, R. Niessner, Formation of polychlorinated dibenzo-p-
dioxins and polychlorinated dibenzofurans during the photolysis of
pentachlorophenol-containing water, Environ. Sci. Technol. 28 (1994)
1145–1149.
[17] F.B. Li, X.G. Wang, Y.T. Li, C.S. Liu, F. Zeng, L.J. Zhang, M.D. Hao, H.D. Ruan,
Enhancement of the reductive transformation of pentachlorophenol by
polycarboxylic acids at the iron oxide-water interface, J. Colloid Interf. Sci. 321
(2008) 332–341.
[33] R.W. Matthews, The reaction chemistry of aqueous ferrous sulfate solutions at
natural pH, Aust. J. Chem 36 (1983) 1305–1317.
[34] T.E. Graedel, M.L. Mandich, Kinetic model studies of atmospheric droplet
chemistry 2: Homogeneous transition metal chemistry in raindrops, Geophys.
Res. 91 (1986) 5205–5221.
[35] T.J. Hartwick, The rate constant of the reaction between ferrous ions and
hydrogen peroxide in acid solution, Can. J. Chem. 35 (1957) 428–436.
[36] C. Walling, Fenton’s reagent revisited, Acc. Chem. Res. 8 (1975) 125–131.
[37] J. He, W.H. Ma, J.J. He, J.C. Zhao, J.C. Yu, Photooxidation of azo dye in aqueous
dispersions of H2O2/a-FeOOH, Appl. Catal. B Environ. 39 (2002) 211–220.
[18] H.K. Young, R.C. Elizabeth, Dechlorination of pentachlorophenol by zero valent
iron and modified zero valent irons, Environ. Sci. Technol. 34 (2000)
2014–2017.